Semiconductor devices have been manufactured by a wet etching method, which is performed by: uniformly coating a photoresist on a conductive metal layer, an insulating layer or a low-k layer which is formed on a substrate such as a silicon wafer or the like by CVD deposition or the like; forming a photoresist pattern by performing selective exposure and development thereon; forming a fine circuit by selectively etching the conductive metal layer or the like formed by the deposition or the like while using the photoresist pattern as a mask; and removing an unnecessary photoresist layer by peeling solution. Recently, however, a dry etching method adapted for a higher-density fine etching is mainly used along with the trend for high density of an integrated circuit.
The dry etching method enables a fine etching compared to the wet etching method, but has a drawback in which the photoresist film is easily deteriorated by the dry etching. The deteriorated film is not easily peeled off by the ashing process for removing a resist, so that a residue of the resist is apt to be generated.
Conventionally, a method for injecting oxygen or oxygen radicals to a resist has been used in the ashing process. In this method, the oxygen radicals react with an organic resist material while being heated, thereby oxidizing carbon and hydrogen in the resist. Then, gaseous reaction products are removed by volatilization.
Recently, however, a material having a low dielectric constant (low-k material) than that of silicon oxide is being widely used for an interlayer dielectric of a semiconductor device. The low-k material is made of polymer containing silicon (Si), oxygen (O), carbon (C) and hydrogen (H). However, the low-k material has low ashing resistance. This is because organic components C and H become gases and removed by the reaction between the oxygen radials and the low-k material, whereby a dielectric constant increases. Accordingly the original purpose of using the low-k material is not achieved.
Therefore, in order to reduce damage of a low-k material in an ashing process, an ashing process using hydrogen radicals has been recently implemented. In this ashing process, hydrogen radicals react with a resist at a high temperature of about 250° C. or higher, thereby cutting carbon bonds (C═C or C—C) in the resist material and producing hydrocarbons of low molecular weight, which will be volatilized as gases.
As for the plasma ashing apparatus using hydrogen radicals, Japanese Patent Laid-open Application No. S63-260030 discloses therein a plasma ashing apparatus for performing an ashing process by providing on a substrate a plasma of a processing gas introduced into a vacuum chamber, the plasma being converted from the processing gas by application of a high frequency to an electrode in a plasma generating chamber provided above a substrate mounting table installed in the vacuum chamber.
However, the plasma ashing process is disadvantageous in that charged particles existing in the plasma inflict damages on devices of a substrate or cause metal contamination. To that end, Japanese Patent Laid-open Application No. 2000-294535 suggests a high-density down-flow type ashing apparatus capable of preventing ions from being directly irradiated to a target substrate by separating a plasma generating chamber from a substrate processing chamber.
The down-flow type plasma ashing apparatus can also be used as an etching apparatus as well as an ashing apparatus. In this apparatus, the substrate is heated by heat transfer from a mounting surface heated by a heating unit buried in a substrate mounting table.
The ashing by the above down-flow type plasma ashing apparatus using hydrogen radicals is advantageous in that a low-k material is less damaged, but is disadvantageous in that a residue of a resist is apt to be generated. The generation of the resist residue is thought to be due to fluorocarbon (CF) polymers generated by an etching process and then deposited on a resist surface, quality deterioration of the resist from the heat during the etching process or the like.
The inventors of the present invention have conducted investigation to explain a mechanism for generating the resist residue and suggest a solution thereto. Hereinafter, the investigation result will be described with reference to accompanying drawings.
FIGS. 4A to 4D show schematic views of images captured by an electron microscope used in monitoring the changes in the resist layer occurring by heating the plasma-etched target substrate at 300° C. for about 20 to 30 seconds.
As shown in FIG. 4A, it was found that a deteriorated layer 3 is formed on a surface of a resist layer 2 on a substrate 1 by the effects of the plasma etching process or a post heat treatment thereof. As a result of various analyses performed on the deteriorated layer 3, the following conclusion has been reached: the resist layer is softened and melts by exposure to a high temperature during the plasma etching process, and carbonization takes place by separation of hydrogen from the softed molten resist layer by an etching gas. As a consequence, a hardened layer is formed on a resist surface. Namely, the deteriorated layer 3 is formed by the effects of heat and the separation of hydrogen by the etching gas. Moreover, it was found that a thickness of the deteriorated layer 3 is about several tenth of a total layer thickness of the resist layer 2. It was also found that parts of the deteriorated layer 3 has several portions that are not flat but are locally wavy as shown in FIG. 4A.
Here, it is considered that the deteriorated layer 3 is thought to be transformed in a wavy shape by a following mechanism. Upon completion of the plasma etching process, the target substrate is generally ashed at a high temperature higher than or equal to 250° C. Accordingly, a temperature difference occurs between a lower portion of the target substrate and a surface of the target substrate, as depicted in FIG. 4B. For example, the lower portion has a temperature of about 100° C., and the surface has a temperature of about 300° C. Besides, the deteriorated layer 3 is different in quality from the original resist, so that a thermal expansion coefficient of deteriorated layer 3 is different from that of the original resist. Therefore, if the target substrate is heated at a high temperature, the surface thereof is compressed by thermal expansion, whereby the deteriorated layer 3 has a wavy shape, as shown in FIG. 4A.
As a result of monitoring the deteriorated layer 3 thoroughly, it was found that there exists a plurality of portions where the deteriorated layer 3 is locally thick as shown in FIG. 4C or where a cavity 4 is formed between the deteriorated layer 3 and the resist layer 2 as shown in FIG. 4D.
In the abnormally shaped portions (thick portions or cavities) of the deteriorated layer 3, it is difficult for hydrogen radicals to diffuse during the ashing process. Therefore, it is believed that the hydrogen radicals are suppressed from penetrating into the resist layer under the abnormally shaped portions. Accordingly, in the abnormally shaped portions of the deteriorated layer 3, the hydrogen radicals may not reach a deep portion of the resist layer, thus hindering cutting of carbon bonds by the hydrogen radicals. These effects are considered to cause the increase of the ashing time and the generation of the resist residue.
The resist residue is considered to be mainly made of amorphous carbon due to the reason described below. When the cutting of carbon bonds by the hydrogen radicals is not effectively made, long-chain hydrocarbon molecules remain on the target substrate without turning into a gas, and then are thermally decomposed by heating at a high temperature during the ashing process. As a result, only hydrogen is removed as a gas, leaving carbon chains. The resist residue thus generated deteriorates the performance of semiconductor devices and causes a poor production yield.
Therefore, if the penetration of hydrogen radicals is facilitated by suppressing the formation of the deteriorated layer 3, it is possible to reduce the resist residue and shorten the ashing time. The inventors of the present invention have made the present invention as a result of various studies to provide a simple and practical device for changing a property of the deteriorated layer 3.